The Effect of Atmospheric Cooling on the Vertical Velocity Dispersion and Density Distribution of Brown Dwarfs

TL;DR

This paper uses Monte Carlo simulations to study how atmospheric cooling affects the vertical velocity dispersion and density distribution of brown dwarfs, revealing spectral type-dependent trends and potential observational signatures.

Contribution

It introduces a simulation-based analysis linking brown dwarf cooling to their kinematic and spatial distribution, highlighting a predicted minimum in velocity dispersion at the L/T transition.

Findings

Velocity dispersion varies with spectral type, showing a minimum at L/T transition.

Predicted trends in disk thickness mirror velocity dispersion patterns.

Cooling influences kinematic properties, deviating from constancy near the hydrogen-burning limit.

Abstract

We present a Monte Carlo simulation designed to predict the vertical velocity dispersion of brown dwarfs in the Milky Way. We show that since these stars are constantly cooling, the velocity dispersion has a noticeable trend with spectral type. With realistic assumptions for the initial-mass function, star-formation history, and the cooling models, we show that the velocity dispersion is roughly consistent with what is observed for M dwarfs, decreases to cooler spectral types, and increases again for the coolest types in our study ($\sim$T9). We predict a minimum in the velocity dispersions for L/T transition objects, however the detailed properties of the minimum predominately depend on the star-formation history. Since this trend is due to brown dwarf cooling, we expect the velocity dispersion as a function of spectral type should deviate from constancy around the hydrogen-burning limit. We convert from velocity dispersion to vertical scale height using standard disk models, and present similar trends in disk thickness as a function of spectral type. We suggest that future, wide-field photometric and/or spectroscopic missions may collect sizable samples of distant ($\sim\!1$ kpc) of dwarfs that span the hydrogen-burning limit. As such, we speculate that such observations may provide a unique way of constraining the average spectral type of hydrogen-burning.